Dialysis and electrodialysis

Dialysis is usually a slow process, the speed depending on the concentration gradient between the sides of the membrane. To hasten the transfer, fresh water usually is passed over one side. As an example, suppose a protein extract needed to have the trace metal ions removed. They are there in a low concentration even at the start. The protein solution or suspension is placed in a dialysis tube, closed off, and suspended in pure water. As the metal ions transfer, they will reach equilibrium, and the transfer will stop. If however, the water is continuously changed, the transfer will continue. It is best to change a small volume of water often rather than dialyze in one large volume. 

The fact that dialysis is slow does not mean that it cannot be used efficiently or quickly. For example, the Autoanalyzer contains a dialyzer unit for biological materials. If you pass the sample through the dialyzer coils, only a few percent of the ions or molecules transfer before the sample passes through. However, this is a very reproducible few percent, and it is easy to quantitate these materials. 

A cleaning procedure recommended by R.F. Boyer Modern Experimental Biochemistry, Addison-Wesley Publishers, 1986, Reading, MA) is, Soak the tubing in 1% acetic acid for 1 hour then transfer to glass-distilled water. To remove metal ions, the tubing is then boiled in basic EDTA solution (1% NajCOj, 10 M EDTA) for 1 hour. Repeat with fresh EDTA. Rinse in hot distilled water five times. Store in distilled water at 4 °C with either a few drops of CHClj or 0.1% NaNj as a preservative.  

Tubing usually comes as a flattened tube 10 m long and 10 to 120 mm wide. Caution This is not the size of the opened tube. A 10 mm flat will produce a 6.4 mm diameter tube and a 16 mm flat will produce a 10 mm tube. Flat sheets are available, 200 x 200 mm. 

Cellulose esters should not be used with acetone, methyl ethyl ketone, or dioxane. 

---

Lacey, R. E. In Handbook ofSeparation Techniques for Chemical Engineers, Schweitzer, P. A. (ed.) 2nd edn. (McGraw-Hill, New York, 1988). Dialysis and electrodialysis. 

Physical Methods of Organic Chemistry", Interscience, NY, Vol 3(1950), pp 313-61 (R.E. Stauffer, "Dialysis and Electrodi-alysis ) 3) Kirk Othmer, 5(1950), pp 1-26 (F.K. Daniel P Stamberger, "Dialysis and Electrodialysis") 4) Perry (1963), Sect 17-42ff ( "Dialysis and Electrodialysis ")... 

Membrane processes such as RO, NF, dialysis and electrodialysis have become recently developed methods for F removal from drinking waters [6,8,15,76-78] and brackish waters [7,9,79], The second part of this chapter is dedicated to the presentation of recent results obtained from the comparison of RO and NF membranes processes for a selective defluoridation of a Senegalese brackish water from the endemic region of Fatick. 

Klein E, Ward RA, and Lacey RE. Membrane processes—dialysis and electrodialysis. In Rousseau RW, Ed. Handbook of Separation Process Technology, John Wiley Sons, Wiley-Interscience, New York, 1987, pp. 954-981. 

Dialysis and electrodialysis have been used in the purification of some polyuronides. " Inorganic impurities may be reduced in amount... 

Section 4.15 describes membranes and introduces a range of membrane separation options. Molecular geometry is exploited in separations of gases via gas permeation. Section 4.16. Dialysis and electrodialysis are considered in Sections 4.17 and 4.18 respectively. Other methods to separate species in liquids are given in Section 4.19, pervaporation Section 4.20, reverse osmosis Section 4.21, for nanofiltration Section 4.22, for ultrafiltration Section 4.23, for microfil-tration and Section 4.24 for chromatographic separations. Separations of larger sized species are considered heterogeneous systems and are considered in Chapter 5. 

Electrochemical cells (electrolysers, batteries and fuel cells) require separators, which allow a flow of specific ionic charges but prevent the transfer of chemical species which remain located either in the cathodic or in the anodic compartment. Among the various separators of electrochemical cells, the ion permeable organic membranes are also used for separation processes such as dialysis and electrodialysis. 

The comments on reverse osmosis apply equally well to dialysis and electrodialysis with selective membranes. 

---